How to add values to only certain index in Numpy 2D Matrix

user_17030072 0
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I have a 2d Matrix

matrix = np.array([[1,2],[3,4],[5,6]])
index = np.array([0, 1, 1])
add_value = np.array([1, 2, 3])

I want to add add_value to matrix but only to the elements corresponding to index in the index list. For example, 1 in add_value should be added to the first element in [1,2], which is 1, resulting in 2. So the output should be

np.array([[2,2],[3,6],[5,9]])
list numpy arrays python
2 Answers

Use a simple multi-dimensional indexing:

matrix[np.arange(matrix.shape[0]), index] += add_value

Or using python builtins:

matrix[tuple(zip(*enumerate(index)))] += add_value

Output:

array([[2, 2],
       [3, 6],
       [5, 9]])
user_163434640 
for i, x in enumerate(add_value):
    matrix[i][index[i]] += x
user_179282980
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